There are two basic groups of capacitor structures available for use in integrated circuit design: junction and thin-film capacitors. This invention is concerned with the thin-film capacitor which is a direct miniaturization of the conventional parallel-plate capacitor.
The thin-film capacitor is composed of two conductive layers separated by a dielectric layer. Thin-film capacitors may be constructed using one of two processes: (a) MOS process using a metal-oxide semiconductor structure; or (b) using a thin-dielectric film between two conductive metal layers. The MOS structure is the most common thin-film capacitor in monolithic circuits because it is readily compatible with the conventional processing technology and does not require multiple metalization layers and is the process by which the capacitor is used in the present invention or fabricated. The capacitance output per unit area of a thin-film capacitor is equal to the ratio of the permittivity (.epsilon..sub.x) to the thickness T.sub.x of a dielectric layer. An MOS capacitor has either silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) which can be used as the dielectric layer. Although silicon dioxide is more readily available, silicon nitride is preferred whenever the extra processing steps can be justified, since it also has a high dielectric constant.
However, for a capacito to be viable in any environment, particularly the integrated circuit environment, it must have a voltage coefficient in the range of .+-.20 ppm across the voltage range of .+-.20 V with the reference capacitance being evaluated at 0 V with a dc bias. The voltage cofficient of a capacitor is the amount of change of capacitance for a given change in voltage. It is desirable that the voltage coefficient be constant at all times and be as close to 0 as possible; however, a coefficient in the range of .+-.20 ppm is a remarkable improvement over what was discovered to be the variable voltage coefficient for a standard poly-metal thin-film capacitor of the prior art.
It will be subsequently pointed out the voltage coefficient of the prior art standard poly-metal capacitors varied over a range of .+-.350 ppm when biased with a voltage over a range of .+-.20 V. This wide range of variability of the voltage coefficient is undesirable and unacceptable in the fabrication of integrated circuits.